It consisted of minicomputers called Interface Message Processors ('IMPs', for short) running specialized packet switching code, joined together with synchronous serial lines; host computers were connected to the IMPs by special bit-serial interfaces (see below).
The protocol family used on the early ARPANET included:
- The Host-to-IMP Protocol, the bottom layer; specified in BBN Report #1822 (multiple editions over time)
- ICP, Initial connection protocol
- Host-to-Host Protocol (early version here)
- FTP (early version here)
- MAIL (here is a much later proposal which eventually became SMTP)